Traditionally, a wafer as a substrate is subjected to various processing steps to manufacture semiconductors. With further advancement in higher integration of elements and miniaturization of circuits in recent year, there is a demand for maintaining a high cleanliness level of environment surrounding wafers, so as to avoid adhesion of particles or moisture on the surface of a wafer. Further, in order to avoid a change in the characteristics of the surface, such as oxidation of a wafer surface, a wafer-surrounding environment is made an atmosphere of nitrogen which is an inert gas, or made vacuum state.
To properly maintain the atmosphere around the wafer, the wafers are placed inside a sealed storage pod called FOUP (Front-Opening Unified Pod), and nitrogen is filled in the inside thereof of the pod. Further, for transferring wafers between a processing apparatus which processes wafers and the FOUP, an EFEM (Equipment Front End Module) is used. The EFEM structures a wafer conveyance chamber which is substantially closed inside a casing. One of wall surfaces facing each other has a load port that functions as an interface unit with the FOUP, and the other wall surface is connected to a load lock chamber which is a part of the processing apparatus. Inside the wafer conveyance chamber, a wafer conveyance apparatus configured to convey wafers is provided. Wafers are loaded and unloaded between the FOUP connected to the load port and the load lock chamber. Usually, the wafer conveyance chamber constantly supplies downflow which is a clean atmospheric air, from a fan filter unit disposed above the conveyance chamber.
However, even oxygen, moisture and the like contained in the atmospheric air used as the downflow could change the characteristics of the wafers, in cutting edge processes of recent years. Under the circumstances, there is a demand for practical application of a technology for circulating an inert gas in the EFEM such as one disclosed in PTL 1.